Technical Field
The present invention relates to an optical recording medium, and more particularly to a write-once optical recording medium containing organic coloring matter serving as a recording material thereof.
Since the age of information has come about, there has been an increasing need for large-capacity memories for recording information, such as images, voice and data, in a large quantities.
To meet the above-mentioned need, disc-shaped optical recording mediums have been widely used because of their various advantages of great recording capacity, satisfactory reliability because recording and reproducing operations are performed in a non-contact manner, portability, low cost and mass production.
As a recording material of the optical recording medium, a variety of materials have been suggested which are a thin film of a rare earth element-transition metal amorphous alloy, such as Tb-Fe-Co, a phase change material, such as Ge-Sb-Te, organic coloring matter, such as a cyanine dye, and other materials. The organic coloring matter is employed in a write-once optical recording medium which permits a user to perform a one time a writing operation. Since the organic coloring matter has no corrosiveness and little toxicity, the organic coloring matter has an advantage that environmental load can be reduced.
A specific structure of an optical recording medium including the organic coloring matter will now be described.
A typical optical recording medium has an air-sandwich structure in which two light transmissive substrates each having a recording layer containing the organic coloring matter formed thereon are bonded to each other in such a manner that the recording layers are disposed opposite to each other while an air layer is interposed between the two substrates. The optical recording mediums having the air-sandwich structures are commercially available as data recording mediums.
A has been suggested in "Proceeding of SPIE", Vol. 1078, pp. 1078, issued in 1989, in which a recording layer containing an organic coloring matter is inserted into a layer structure of a usual compact disk (CD) formed by sequentially forming, on a light transmissive substrate, a recording layer containing organic coloring matter, a light reflecting layer and a protective layer. The above-mentioned optical recording medium has a high reflectance not lower than 70% when a wavelength of 780 nm is use with the compact disk. Therefore, the foregoing optical recording medium attains a signal characteristic compatible with commercial compact disks after data has been recorded on the optical recording medium. The optical recording mediums have been used to record musical sound, images and data for personal computers and have garnered a substantial share in the market.
In the optical recording industrial field, optical systems for optically recording data have been researched and developed in addition to the recording materials and the structures of the mediums.
Data is recorded on the optical recording medium and reproduced from the same by irradiating the optical recording medium with a laser beam. When a recording operation is performed, laser beams are converged to the surface of a recording layer. Thus, the recording layer is optically changed within the formed laser spot so that pits are formed. When a reproducing operation is performed, laser beams are converged to the pit so as to detect the difference in the reflectance from a region in which no pit has been formed. The density at which data can be recorded on the optical recording medium is determined by the diameter of a laser spot formed by a laser beam. In inverse proportion to the diameter of the laser spot, the recording density can be raised.
On the other hand, the diameter of the laser spot is in proportion to .lambda./NA (where NA is the number of apertures of an objective lens and .lambda. is the wavelength of the laser beam). The recording density of the optical recording medium is determined by the number of apertures NA of the objective lens having the corresponding optical diameter and the wavelength .lambda. of the laser beam. In proportion to the NA and in inverse proportion to .lambda., the recording density can be raised.
Therefore, research and development for shortening the wavelength of the semiconductor laser beam has been performed energetically. As reported in, for example, "O plus E", vol. 199, pp. 71 (1996), an attempt has been made to use semiconductor laser beams having wavelengths of 630 nm to 680 nm in the optical recording process. A so-called digital video disk (DVD) structured to be capable of obtaining a recording capacity which is six to eight times that of the CD employs a semiconductor laser beam having a wavelength of 635 nm or 650 nm.
Although the organic coloring matter is a preferred recording material for the optical recording medium, there arises a problem in that the coloring matter photo-deteriorates.
The coloring matter photo-deteriorates because the p-electron conjugated system in the chemical structure of the coloring matter is chemically changed and thus the color of the coloring matter is changed. Another fact has been found that singlet oxygen effects the mechanism of the photo-deterioration as well as the chemical change of the coloring matter. The singlet oxygen is generated when energy transfer takes place from an excited which is state realized because the coloring matter has absorbed light, to a normal state.
Therefore, an attempt has been made to employ a chemical substance for preventing the action of the singlet oxygen to prevent the photo-deterioration of the coloring matter.
That is, the singlet oxygen having an electrophilic characteristic attacks unsaturated bonds existing in the organic coloring matter, causing dioxetane to be generated. Moreover, the dioxetane is chemically changed so that the coloring matter is decomposed. Therefore, if a chemical substance which can easily be oxidized as compared with the double bond existing in the organic coloring matter is contained, the chemical substance is oxidized by the singlet oxygen in place of the coloring matter. Thus, exertion of the influence of the singlet oxygen on the organic coloring matter can be prevented. The chemical substance of the type which can easily be oxidized is exemplified by aromatic amine.
If a chemical substance having an exciting energy lower than the exciting energy of the singlet oxygen which is about 1000 cm.sup.-1 is contained, energy transfer takes place from the singlet oxygen to the chemical substance. As a result, the singlet oxygen is returned to the normal state and deactivated. The chemical substance having the low exciting energy is exemplified by a nickel metal complex and a copper complex.
If the organic coloring matter is employed as the recording material, concurrent use of the above-mentioned chemical substance enables satisfactory light resistance to be realized.
However, use of the above-mentioned chemical substance causes the following problems to arise.
When a recording layer is formed by the organic coloring matter, the organic coloring matter is dissolved in a solvent so that a coating material made of the organic coloring matter is prepared. Then, the coating material made of the organic coloring matter is applied to the surface of the disk substrate. Then, the disk substrate is dried. When the chemical substance for preventing photo-deterioration of the coloring matter is employed, the chemical substance is dissolved in the coating material made of the organic coloring matter.
Since polycarbonate resin for making the substrate of the optical disk has a characteristic that it is affected by substantially all solvents except for alcohol, an alcohol solvent is required for the coating material made of the organic coloring matter.
However, a major portion of the chemical substance for preventing photo-deterioration of the coloring matter cannot easily be solved by the solvent. Therefore, it is very difficult to prepare a chemical substance which can sufficiently be solved by the alcohol.
Since the aromatic amine can easily be oxidized, the prepared aromatic amine cannot easily be refined. Thus, high purity aromatic amine cannot easily be obtained. What is worse, the cost cannot be reduced. As a result, the cost for manufacturing the optical disk cannot be reduced.
On the other hand, the chemical substance, such as the nickel metal complex, having the function of deactivating the singlet oxygen has poor absorbance in the laser wavelength region. If the chemical substance of the foregoing type is contained in the recording layer, the refractive index of the recording layer is undesirably changed. As a result, there arises a problem in that a degree of modulation of a signal required to perform reproduction cannot easily be obtained.
As described above, the process for adding the chemical substance for preventing photo-deterioration of the coloring matter has the various problems. Therefore, a substitute for the above-mentioned method has been required. However, another effective method has not been found.